Effect of doping gadlinium atoms into weak itinerant-electron ferromagnets or substituting a part of constituent metal atoms for different transition-metal atoms in weak itinerant-electron ferromagnets have been studied experimentally for ZrZn_2, YNi_3, Y_2Ni_7 ferromagnets. Effect of substituting anions in CoS_2 ferromagnet on its magnetic property has also been examined. Measuremnts have been made of magnetization curves in a very strong magnetic field. The temperature variation of magnetization in the Gd-doped weak itinerant-electron ferromagnets can be explained by taking into consideration of the induced magnetic moment in itinerant-electrons due to localized spins, on the basis of the mean field approximation. However, the magnetic field dependence of magnetization shows an inadequacy of the mean field approximation and suggests the effect of field on spin fluctuations to suppress and also an effect of anisotropy. The substitution by transition-metal atoms appears to shift a Fermi level as a first order approximation, but some complicated band structure effet seems to dominate the variaton of magnetization. The substitution of selenium for sulphur results in a drastic change of magnetic states in the itinerant-electron ferromagnet CoS_2. It appears a change from a ferromagnet to an itinerant-electron metamagnet, and further to a paramagnet by the substitution. Especially, in the 10 % substituted sample, it has been observed a clear first order phase transiton from ferromagnetic state to a metamagnetic state and a second order phase transiton from a metamagnetic state to a paramagnetic state by increasing a temperature. This is due to a quite homogeneous state of our samples. This transiton can be qulitatively analyzed by the Landau's theory of phase transiton. A possible size effect in the very weak itinerant-electron ferromagnets has been examined experimentally, but no appreciable change has been observed.